The present invention relates to a thixotropic amino formaldehyde resin with a urea-functional compound and the use thereof in coating formulations.
Thixotropic agents or sag control agents are essential elements of nowadays-produced coating compositions. They not only provide a stable mixture of the several components in a coating composition, but also enable the application of the coating, even in thick layers on vertically placed objects, without the occurrence of sags in the cured coating.
It is well known in the art to prepare a thixotropic binder composition, e.g., by mixing a binder and a sag control agent. For example, in EP 0,198,519 a diurea compound is used as a sag control agent in a thixotropic coating composition. The preparation of the sag control agent may be carried out in the presence of a binder. In GB 1,454,414 a thixotropizing urea adduct is prepared in situ in the presence of the binder. In this publication it is further said that urea adducts prepared in situ in the presence of an amino formaldehyde resin, readily show incompatibility with the resin. It is concluded that it is more effective to carry out in situ formation of the urea adduct in the plasticizing binder.
In GB 1,454,414 a general method is disclosed for the preparation of a thixotropic amino formaldehyde resin-containing coating composition, viz. in situ formation of a urea adduct in the binder component and then mixing the thixotroped binder with the amino formaldehyde resin. Urea adducts are obtained by reacting an amine-functional compound and an isocyanate-functional compound. Substantially the same method is used in EP 0,198,519 wherein the binder is selected from ester diols, polyesters, polyurethanes, alkyd resins, acrylate, and methacrylate resins. In DE 1,805,693 the formation of urea-functional compounds is described without the presence of any binder. The thus obtained thixotropic coating compositions based upon a thixotropic binder and an amino formaldehyde resin are well known and used in the art.
However, in practice a large number of thixotropic coating compositions are used containing different binders. As a consequence the different binders should all be modified with a thixotropic agent. Therefore, the use of a universal thixotropic amino formaldehyde resin would give a substantial advantage in the preparation of coating compositions.
Moreover, since in situ formation of a urea adduct in the binder component limits the functionality of the binder component, e.g., the binder component should not be reactive towards the amine-functional compound or the isocyanate-functional compound which are used for the in situ formation of the urea adduct, there is a need for a thixotropic coating composition based upon a thixotropic amino formaldehyde resin.
Surprisingly, it was found that a thixotropic amino formaldehyde resin with a urea-functional compound is obtained by adding a polyisocyanate-functional compound to a monoamine-functional compound or a monoisocyanate-functional compound to a polyamine-functional compound in the presence of the amino formaldehyde resin and reacting the amine and isocyanate-functional compounds to form the urea-functional compound entangled as solid material in the resin. The thus obtained urea-adduct acts as a rheology modifier and a thixotropic effect is developed in the amino formaldehyde resin.